Image compression systems based on linear transforms such as the discrete cosine transform (DCT), sub-band coding, and wavelet transforms, have been extensively studied since the early 1970s. These compression systems fall into broad categories, Interframe systems and intraframe systems. An exemplary Interframe system is that developed by the Motion Picture Experts Group (MPEG). An exemplary intraframe system is that developed by the Joint Photographic Experts Group (JPEG). For the most part research on these image compression systems has been focused on applications such as multimedia and high definition television which require only single generation compression and decompression.
Recently a need has arisen for compression systems that can pass an image through the compression decompression cycle many times without appreciable degradation. This type of multi-generational compression and decompression is a severe test of any type of compression system because coding artifacts may be amplified in the higher generational copies. Several applications would benefit from a compression technique which would not exhibit degradation through multi-compression and decompression cycles. These include the editing a program recorded on a digital tape recorder, the distribution of video signals in a studio or network, and the compression of high definition television signals for recording on a digital tape recorder.
A particular goal for such an image compression system is to allow the recording of high definition television (HDTV) signals, which are provided in a format such as YUV422 format having 8 bits per sample or 16 bits per pixel, on an existing digital tape recorder such as a D3 or D5 recorder. To meet the bit rate requirements of a D3 or D5 recorder however, the HDTV signal would have to be compressed to have a bit rate of about 2-bits per pixel and 4-bits per pixel, respectively.
In addition, the compression system desirably should support standard and trick play modes in the forward and reverse directions and should also support a "picture-in-shuttle" mode in which a sped-up image is displayed during fast forward and reverse operations.
It is also desirable for the data to be able to be edited on a frame by frame basis. Editing operations of this type may be as simple as changing the contents of single frames, for example inserting text into an image. It may, however, involve more complex processing such as cutting between two scenes, inserting one image into a sequence of other images, and merging two or more scenes, as, for example, in a fade between two image sequences.
It is also desirable that such a data compression system allow an image sequence to be compressed and decompressed many times without significantly affecting its quality.
Finally, it is desirable for the compression system to operate in real-time or near real-time. This requirement implies that the image compression system be implemented either entirely in hardware or in a combination of dedicated hardware and software for a high speed general purpose image processing system.
These requirements, especially the requirements of frame-at-a-time editing and forward and reverse play at different speeds would be difficult, if not impossible, to implement using an Interframe coding system such as MEPG. Thus, the coder is desirably an intraframe coder which can achieve the desired compression ratio.
As described above, relatively efficient intraframe systems exist which are based on linear transforms. The JPEG compression system, for example, is based on discrete cosine transforms. Systems based on linear transforms, however, are not well suited for multi-generational compression and decompression. Linear transforms have difficulty compressing sharp edges. Consequently, these edges tend to blur and soften through multiple compression and decompression operations. In addition linear transforms introduce arithmetic round off errors and quantization errors. Consequently, the general signal to noise level tends to decrease during multi-generational compression and decompression due to the accumulation of these errors. In addition, spatial shifting due to editing may introduce new groupings of pixels that may prevent the numerical errors from stabilizing.